Are these fish compatible? I know with different betta's and their personalities its hard to say exactly, I just want to see if anyone has any experience with this combo.
I don't know about the dwarves, but I keep my betta with Australian rainbows (M. splendida) without issue. These rainbows are very active and prefer the side of the tank with more current, where my betta likes things quieter, he's claimed the plants on the slow side as his turf. I also have to feed the rainbows first, they're voracious eaters and will take his food if they aren't already distracted. There's never been any agression or any kind of nipping issue, so if the dwarf rainbows are tempermentally similar to the Australians...if you can create terretories that accomodate the different current preferences, I think you'll be fine .
I tried to have a betta in a tank which had neon dwarf rainbows and they didn't seem incompatible but since I had pearl gouramis these almost killed the betta and I took him out. But I think you would have no problem with that combination. Just watch if the betta is able to eat, though I think that would not be an issue either.